The present invention relates to a metal container can lid, more particularly to a metal easy-open can lid which enables part or substantially all of the can lid to be easily opened manually and to a process for producing the same. This can lid is used for beverage cans or general food cans and a wide range of other applications.
The easy-open can lids (or easy-open can lids) used for beverage cans, general food cans, etc. are made of surface-treated aluminum sheet or steel sheet provided with score lines (or guide lines for opening) for tearing of the can lid to form an opening for removing the contents of the can.
At present, the method for forming a score line, as shown in FIG. 1, is to shape the can lid material 1 into the basic form of the can lid, then placed it on a flat die 2 and press down a die 3 having a projection of the shape of the contour of the opening to form a guide groove for opening (score) 4 of the shape of the opening in the material. To facilitate the can openability, it is necessary to press the die down until the score depth reaches about xc2xd to ⅔ of the thickness of the sheet before processing. However, if the depth of the score is too shallow, the can openability becomes poor, while if too deep, the strength becomes insufficient and the problem arises of the can opening in transport by a small impact from the outside. Therefore, there have been the problem that the projection of the die for forming the score is required to have a high precision in the shape of the angular tip of the projection and the shape of the projection of the die changes and deteriorates due to wear when forming the score.
On the other hand, as shown in FIG. 2, as in Japanese Unexamined Patent Publication (Kokai) No. 6-115548, Japanese Unexamined Patent Publication (Kokai) No. 8-224626, and Japanese Unexamined Patent Publication (Kokai) No. 9-108756, the process of production of an easy-open can lid by pressing the can lid material 1 by the shoulder portions of punch shoulder 8 and die shoulder 9 of upper and lower dies of an upper die (i.e., punch) 5 and lower die (i.e., die) 6 to form the score line has been proposed. However, according to this method, since the contour of the score line is generally asymmetric, the deformation of the material at the time of pressing did not become uniform and it was difficult to obtain a desired distribution of thickness across the entire length of the score line. That is, if trying to control the thickness of the thinnest portion of the score line (residual thickness) to be thin so as to improve the can openability, partially broken or extremely thin portions will be formed resulting in leakage of the contents and occurrence of unnecessary opening due to impact at the time of transport etc. Further, if trying to control the residual thickness to be thick in order to avoid breakage or occurrence of extremely thin portions, the problem will remain that part of the score line will not tear even after an opening operation, and therefore, the can openability will be insufficient.
For the formation of the score of commercialized easy-open can lids, a scoring blade having an acute cross-section is used. Further, even in the push-tab type, the tear strip is cut using a sharp cutting blade. There are problems in these in terms of productivity from the viewpoint of the tool life. Further, particularly when using steel sheet as a material, the surface coating is broken by the processing and the metal is exposed, so repair coating is required. Further, when it comes to recycling, even among metal cans, while so-called xe2x80x9cmonometal cansxe2x80x9d where the can body and the can lid are formed by the same material can be said to be suitable for recycling, the present easy-open can lids are almost all made of aluminum. On the other hand, the bodies and can lids other than easy-open can lids are almost all made of steel sheet. Therefore, there has been a fervent desire for the development of some means enabling production, with a good productivity, of easy-open can lids made of steel sheet which are superior in can openability and superior in corrosion resistance.
An easy-open can lid in which a score line is formed by pressing the above resin laminated metal sheet by the shoulder radii of upper and lower dies is meant to solve the above-mentioned problem, but practical problems remain in regard to the can openability and corrosion resistance.
As explained above, even with the method of pressing the shape of an opening by the shoulders of the upper and lower dies proposed for solving the change and deterioration of the shape of the projection of the die due to wear at the time of scoring, there are still problems remaining in the score line formed in the easy-open can lid due to the processing precision such as how to simultaneously achieve can openability, air-tightness, and impact resistance. The present invention achieves higher precision of control of the residual thickness at the time of forming the score line by pressing by die shoulders.
In accordance with the present invention, there is provided a metal easy-open can lid superior in can openability having a resin coating on at least one surface thereof, wherein said easy-open can lid has an outer peripheral flat portion and an inner peripheral flat portion straddling a score line further having an outer bead and inner bead at the sides of the outer peripheral flat portion and the inner peripheral flat portion opposite to the score line, and having a cross-section where the score line becomes gradually thinner from the thicknesses of the outer peripheral flat portion and the inner peripheral flat portion.
The easy-open can lid superior in can openability can have a cross-sectional shape where the gradually thinned thinly formed portion is bent.
In accordance with the present invention, there is further provided an easy-open can lid superior in can openability wherein at least the inner bead is shaped projecting to the can outside, the deepest recess point toward the can inside of the inner peripheral flat portion is positioned toward the can inside in a range of 0.2 to 4.0 mm at the can outside surface compared with the highest projecting point toward the can outside of the inner bead, the interface portion of the outer peripheral flat portion and the score line is positioned toward the can outside in a range of 0.1 to 2.0 mm at the can outside surface compared with the deepest recess point toward the can inside of the inner peripheral flat portion, and the widths of the inner peripheral flat portion cross-section and the outer peripheral flat portion cross-section are respectively from 0.1 to 3.0 mm.
The easy-open can lid can be made of a metal sheet such as steel sheet, surface-treated steel sheet, aluminum sheet, or aluminum alloy sheet. The resin coating on the surface of the metal sheet at least at the can inner surface side can be made by a thermoplastic saturated polyester-based resin of a thickness of 10 to 100 xcexcm. Further, the resin coating corresponding to the can outer surface can be made with a polyamide resin having a thickness of 10 to 100 xcexcm.
The easy-open can lid according to the present invention, is either of the types where can lid does has not a tab for opening a tear strip provided at least at one location of the can lid separating part or all of the can lid from the can body and the can lid does has not a tab for opening a tear strip provided at least at one location of the can lid leaving part or all of the can lid on the can body.
Further, the easy-open can lid may be opened by the method of opening a tear strip provided at least at one location of the can lid, without a tab, leaving part of the tear strip at the can body or separating it from the can body.
In accordance with the present invention, there is provided a method of forming an easy-open can lid superior in can openability comprising, when pressing the shape of the opening by shoulders of upper and lower dies, pressing by using an upper die (punch) and lower die (die) provided with recesses at the die centers for restraining the shaping material at the lower die and/or upper die, having at least one of the punch shoulder R and die shoulder R of not more than 2.0 mm, and having the punch and die overlap on a parallel line of operation of the punch.
In the above method of formation, preferably the depth of the recess of the lower die (die) is not more than (punch shoulder R+die shoulder R)xc3x974 and the depth of the recess of the upper die (punch) head is at least 0.2 mm.
In accordance with the present invention, there is further provided a method of forming an easy-open can lid superior in can openability comprising, when pressing the shape of the opening by shoulders of upper and lower dies, pressing by using upper and lower dies provided with recesses at the die centers for restraining the shaping material at the lower die and/or upper die, having a shape of the shoulders of the die and the punch of part of an ellipse having a long radius of 0.1 to 5.0 mm and a short radius of 0.05 to 4.0 mm, and having the punch and die overlap on a parallel line of operation of the punch.
In the above method of formation, preferably the depth of the recess of the lower die (die) is not more than (punch shoulder R+die shoulder R)xc3x974 and the depth of the recess of the upper die (punch) head is at least 0.2 mm.